This application is based upon and claims priority of Japanese Patent Application No. 2000-001861, filed on Jan. 7, 2000, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to magnetic elements using a ferromagnetic tunnel junction, and magnetic memory devices comprising such elements.
2. Description of the Related Art
An MRAM (Magnetic Random Access Memory) is one of a memory device such as a DRAM (Dynamic Random Access Memory) and an SRAM (Static Random Access Memory) using semiconductor substances. While the DRAM and the SRAM store data in accordance with the presence/absence of electric charges, the MRAM stores data in accordance with the magnetized directions of magnetic substances. MRAM has many merits, e.g., it is suitable for high speed operation, it shows high radiation resistance, and it shows little deterioration due to repetition of data write operations. For this reason, the study of the MRAM has been prosecuted earnestly in recent years.
The MRAM has its basic structure in which magnetic memory elements are arranged into a matrix, word and bit lines are disposed in columns and rows for generating a magnetic field in a selected element, and terminals are provided for reading out data stored in the selected element. When one of the word lines and one of the bit lines are selected, and electric currents are applied to the selected word and bit lines, a magnetic field is generated in the magnetic element at the intersection of the selected word and bit lines. The magnetized direction of the magnetic element can be reversed by the magnetic field. In this manner, two different magnetized states of each magnetic element can be realized. The two magnetized states correspond to bit data of xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, respectively.
For the memory structure of each magnetic element of an MRAM, usable are so-called MR (Magneto-Resistive) element, GMR (Giant Magneto-Resistive) element, and ferromagnetic tunnel element, any of which changes in its electric resistance in accordance with magnetized directions. To read out data from a magnetic element, an electric current is applied to the element and the electric resistance thereof is measured.
MRAM using an MR or a GMR element for the memory structure of each magnetic element has been realized. In this type of MRAM, however, the sheet resistance of each magnetic element is measured. Therefore, if the element size is reduced, the resistance to be measured is reduced accordingly, so that the output is reduced. Although reduction in size of such an element is a recent general demand, this type of MRAM has its limit.
A ferromagnetic tunnel element has a tunnel junction structure generally comprising a ferromagnetic layer, an insulating layer, and another ferromagnetic layer laminated in this order. In this structure, the tunnel resistance in cases when both the magnetic layers are magnetized in the same direction differs from those cases when magnetized in reverse directions. The amount of change in resistance depends on the polarizability of each magnetic layer. A change in resistance by more than 40-50% is expected if a suitable material is chosen. Besides, the smaller the junction area is, the higher the tunnel resistance is. Thus the element size can be reduced without reducing the output.
For these reasons, use of such a ferromagnetic tunnel junction structure for the memory structure of each magnetic memory element of an MRAM is expected to realize a higher packing density in the MRAM.
This idea for improving the packing density in an MRAM, however, includes the following vital problems.
First, each of the word and bit lines of an MRAM must receive an electric current for generating sufficient magnetic field that can reverse the magnetized direction of the target magnetic layer of each memory structure. For this reason, each of the word and bit lines requires a certain degree of size in its cross section. This requirement in size limits the packing density. To avoid this problem, the electric current for generating the magnetic field must be made small. This requires selection of a suitable magnetic material whose magnetized direction can be reversed with a weaker magnetic field.
Second, if the size of such a memory structure using ferromagnetic tunnel junction is reduced, both the ferromagnetic layers sandwiching the insulating layer may be magnetostatically coupled through leakage fluxes out of the ferromagnetic layers. This lowers the sensitivity of the memory element to an external magnetic field.
FIG. 1 shows such a magnetic memory structure. Referring to FIG. 1, ferromagnetic layers 101 and 103 sandwich an insulating layer 102. In this structure, the ferromagnetic layers 101 and 103 may be magnetically coupled with each other, so that leakage fluxes become very few. This causes a bad sensitivity to an external magnetic field.
FIG. 2 shows a more specific structure of a magnetic memory element in this type of MRAM. In this example, a ferromagnetic layer 101 (thickness: 2.0 nm) made of CoFe, an insulating layer 102 (thickness: 1.5 nm) made of Al2O3, and a ferromagnetic layer 103 (thickness: 1.0 nm) made of CoFe are laminated in this order on a magnetic underlayer 104 (thickness: 10 nm) made of IrMn. In this structure, external fluxes maybe made from an end of the CoFe layer 103 whose magnetized direction is fixed (hereinafter referred to as fixed layer), to an end of the CoFe layer 101 whose magnetized direction is to be reversed (hereinafter referred to as free layer). This causes very bad sensitivity of the CoFe layer 101 to an external magnetic field.
Besides, magnetic domain structure is a factor of stability of such a magnetic element. In general, a magnetic substance comprises a number of magnetic domains having the same magnetized direction. The boundary between such domains is called a magnetic domain wall. In case of a magnetic element of an MRAM in which a magnetized direction is changed, such magnetic domain walls may move. This causes noise and deterioration of sensitivity.
It is an object of the present invention to provide magnetic elements having a relatively simple construction, being less affected by magnetostatic coupling, and capable of realizing improvement of the sensitivity for reversing magnetized direction by a single domain structure. It is another object of the present invention to provide magnetic memory devices comprising such magnetic elements for realizing less power consumption, high-speed operation, and high packing density.
According to an aspect of the present invention, a magnetic element comprises a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer laminated in this order. At least one of the first and second ferromagnetic layers comprises a lower ferromagnetic layer, a non-magnetic conductive layer, and an upper ferromagnetic layer laminated in this order.
Preferably, the upper and lower ferromagnetic layers sandwiching the non-magnetic conductive layer are antiferromagnetically coupled with each other.
Preferably, the magnetized direction of at least one of the first and second ferromagnetic layers is fixed by an adjacent antiferromagnetic layer.
Preferably, the non-magnetic conductive layer is made of one of Ru and Cu.
Preferably, the amount of magnetization of the upper ferromagnetic layer differs from that of the lower ferromagnetic layer.
Preferably, the thickness of the upper ferromagnetic layer differs from that of the lower ferromagnetic layer.
According to another aspect of the present invention, a magnetic memory device comprises magnetic elements each of which comprises a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer laminated in this order. The magnetized direction of one of the first and second ferromagnetic layers is changeable in accordance with data to be stored. At least one of the first and second ferromagnetic layers comprises a lower ferromagnetic layer, a non-magnetic conductive layer, and an upper ferromagnetic layer laminated in this order.
According to the present invention, at least one of the first and second ferromagnetic layers has the structure that upper and lower ferromagnetic layers sandwich a non-magnetic conductive layer. By properly selecting the kind or composition of the material and the thickness of each of the upper and lower ferromagnetic layers, the amount of magnetization of each of them can be so regulated as to reduce the affection by magnetostatic coupling. Changeability of magnetized direction of the first or second ferromagnetic layer in response to external magnetic field can thereby be adjusted into a suitable value. This affords an improvement of sensitivity.
According to the present invention, realized are magnetic elements having a relatively simple construction, being less affected by magnetostatic coupling, and capable of improving the sensitivity for reversing magnetized direction by a single domain structure. Also realized are magnetic memory devices comprising such magnetic elements for less power consumption, high-speed operation, and high packing density.